Japanese Patent Application Publication No. 2011-34713 (hereinafter referred to as a “Document 1”) discloses a lighting fixture equipped with a visible-light-communication function that transmits a communication signal by using illumination light. The lighting fixture includes: a light source substrate on which a light source is mounted; a lighting circuit substrate which is electrically connected with the light source substrate and performs lighting control of the light source; and a visible-light-communication controlling substrate which superposes a communication signal on outgoing light from the light source. In the lighting fixture, the visible-light-communication controlling substrate is detachably disposed between the lighting circuit substrate and the light source substrate, and a design thereof can be accordingly made common between a device with a visible-light-communication function and a device with no visible-light-communication function. Control electrical power of the visible-light-communication controlling substrate is supplied from an output terminal of the lighting circuit substrate.
However, when a switch device for modulation in the visible-light-communication controlling substrate is turned off, a load voltage onto light-emitting devices (LEDs) in the light source is shut off, and accordingly the lighting fixture of Document 1 is put in a condition without substantial load with respect to the light source. A PFC circuit having a boost function for improving distortion of an input electric current is used for a general power supply circuit of which electric power supply is commercial power AC, and accordingly if the light source is put in the unloaded condition, a voltage across the output terminals of the power supply circuit rises close to output voltage of the PFC circuit. If the voltage across the output terminals of the power supply circuit rises, an input voltage onto a control power supply of the visible-light-communication controlling substrate rises as well, which causes loss of circuit. Therefore, the circuit configuration can be considered in which a resistor for modulation is provided in parallel to a switch device for modulation, for modulating an output current supplied from the power supply circuit to the light source, thereby preventing unloaded condition of the light source even when the switch device for modulation is turned off.
However, in this circuit configuration, the power supply circuit performs constant current control, and accordingly when the switch device for modulation starts an ON operation, an electric current flows through a side of the switch device for modulation, thereby short-circuiting two ends of the resistor for modulation. At this time, as shown in FIG. 8, an output voltage onto the light source slightly decreases, whereas impedance between output ends of the power supply circuit decreases sharply. Accordingly, a surge current is included in an output current to the light source and an excessive current stress instantaneously flows through LEDs of the light source. A maximum rated current of an LED is set to about twice of a rated current in general, but may be set to be lower than that. There is a concern that LEDs suffer damage if the aforementioned surge current generates. If a resistance value of a resistor for modulation is decreased, the occurrence of a surge current can be suppressed. However, this instance causes decrease of a modulation width (modulation ratio) by ON and OFF of a switch device for modulation, which is undesirable for a visible light communication.